This invention relates to a method and apparatus for visualizing the behavior of pressure waves in a liquid-solid system. More particularly, the invention relates to a method and apparatus for visualizing the behavior of ultrasonic waves in a liquid and solid simultaneously for the purpose of flaw detection and the like.
A high degree of safety and reliability is required in the equipment and supporting structures of atomic power stations, thermal power stations and chemical plants, the various mechanical equipment in aircraft and automobiles, the steel-frame construction of buildings and bridges, ceramic materials and the like. To this end, the soundness of equipment and materials is verified periodically by a variety of non-destructive tests, one of which is an ultrasonic test for flaw detection.
FIGS. 9(A) and 9(B) are views illustrating a conventional method of performing an ultrasonic test for flaw detection, in which FIG. 9(A) shows a method of flaw detection by direct contact and 9(B) a method of flaw detection by submersion in water. Numeral 21 denotes an ultrasonic direct contact-type probe, 22 a sample undergoing the test, 23 a flaw, 24 an ultrasonic submersible-type probe, and 25 water.
In the conventional method of direct-contact flaw detection shown in FIG. 9(A), the ultrasonic probe 21 (hereinafter referred to simply as a "probe") is brought into direct contact with the sample 22 to examine the sample for the flaw 23. In the conventional method of submersion-type flaw detection illustrated in FIG. 9(B), the probe 24 irradiates the sample 22 with ultrasonic pulses (hereinafter referred to simply as "pulses") through the medium of the water 25.
The direct-contact flaw detection method using the direct contact-type probe 21 is used for detecting flaws in equipment and materials having a comparatively simple shape, such as flat plates, pipes and the like. The submersion-type flaw detection method employing the submersible probe 24 is in wide use since it is applicable to equipment and materials having a complicated shape and excels in terms of probe scanning capability, stability of coupling characteristics and the like.
The sound field and pulse waveform of ultrasonic pulses emitted by a probe can be examined by (1) measuring the waves reflected from a transverse hole, longitudinal hole or slit-type flaw actually formed in a sample, or from minute reflectors such as spheres immersed in water, or (2) by using a solid model and visualizing the ultrasonic pulses that propagate through the model. The latter method enables the pulse waveform and sound pressure to be measured in detail. In order to visualize the pulses, use is made of the schlieren method, in which the light source is a strobe having a short flash time, or a photoelastic testing method.
Though the method using the minute reflectors is advantageous in that measurements can be mde with ease using a material the same as that of the sample, a drawback is that the reflector has a marked frequency characteristic with respect to ultrasonic waves so that the reflected waveform is changed by the input waveform. This makes detailed measurement impossible.
The schlieren method provides a high visualization sensitivity with respect to pulses in water and makes detailed analysis possible. However, the method is difficult to apply in solids since sensitivity is poor in such a medium.
The photoelastic testing method provides highly sensitive visualization only for pulses in solids and makes quantitative evaluation possible. However, since shearing stresses do not act in water, pulses in water cannot be visualized in theory.
Accordingly, if sound field measurement of pulses from a submersible probe is to be evaluated quantitatively, the schlieren method enables visualization only of the pulses that propagate through the water and not the solid, whereas the photoelastic testing method is capable of visualizing only the pulses that propagate through the solid and not the water. Thus, overall evaluation of pulses in both water and solid is not possible.
It is necessary to ensure uniformity of the quantitative results (i.e. echo height, beam path, etc.) of flaw detection using a probe, where uniformity means the ability to make the same judgements with regard to the same flaw regardless of the probe used, and to decide optimum flaw detection conditions, such as probe selection (frequency, angle of refraction, etc.), placement of the probe and scanning pitch, when actually performing a flaw detection test. In order to obtain uniformity of quantitative results and decide optimum flaw detection conditions, the sound field characteristics of the probe and the pulse waveform are among the most important factors to be decided.
Accordingly, in order to perform an overall performance evaluation and examination of the sound field and pulse waveform of ultrasonic pulses from various types of probes inclusive of the submersible type, there is need for development of a technique that will enable the features of both the schlieren method and photoelastic testing method to be applied simultaneously.